The part of a hip prosthesis assigned to the femur is equipped with a shaft inserted into a cavity which, after resection of the head and neck of the hip, is formed by a suitable tool in the spongy inner cross-sectional area of the femur. If the shaft is inserted without using cement, it is endeavored to make the shape of the cavity as far as possible complementary to the shape of the shaft, so that the prosthesis has a secure and firm fit after insertion of the shaft. The proximal portion of the shaft coming to lie in the metaphyseal region of the bone (approximately above the lesser trochanter) is designed in such a way that it can transmit not only vertical forces in the direction of the femur but also forces extending transverse thereto and, in particular, medially directed forces. To ensure that the surface areas of the proximal prosthesis shaft oriented in the ventral and dorsal directions also take part in transmission of forces to the bone, it is known to design their surface such that a form-fit connection with the bone tissue can be obtained. Two possibilities are available for this, namely, on the one hand, a surface micro-roughness which is brought about, for example, by glass blasting, porous coating or the like, and allows the bone to infiltrate into the depressions and pores, and, on the other hand, ribs which project from the base body of the shaft. These two possibilities can also be used together. Thus, it is known (EP-B-761 183; U.S. Pat. No. 5,755,811) to arrange ribs on the dorsal and ventral faces of the proximal base body of the shaft, said ribs extending in the longitudinal direction of the shaft, and their cross section increasing in a wedge shape from distal to proximal. Such ribs are referred to below as wedge-shaped ribs. The cavity is produced using a rasp whose shape corresponds to the base body of the shaft without the ribs. Upon insertion of the shaft, the base body of the shaft forms a press fit with the surface of the cavity. The wedge-shaped ribs cut into the spongy bone tissue as the prosthesis is being inserted. Because of their wedge shape, they displace and compact the bone tissue. This contributes to the secure fit of the prosthesis. First, a macroscopic form fit is thus obtained between the bone and the prosthesis cross section, by virtue of the rib. Second, a microscopic form fit is obtained after bone tissue has grown into the rough or porous surface structure. These two effects evidently occur independently of one another and are used independently of one another in the construction of a prosthesis.
Of course, the said compression of the bone material during insertion of a wedge-shaped rib must not lead to the bone breaking. This danger is all the greater, the wider a rib, because the latter then directs a greater wedge surface toward the cortical bone and produces a greater wedging force against the cortical bone. Known wedge-shaped ribs are therefore made of a narrow design (EP-B-761 183, EP-B-159 462). For the same reason, it is sought to deflect the wedging action of the ribs into the tangential direction (DE-U-295 22 382, page 4, line 22). It is true that wide ribs are also known in hip prostheses intended for cementless implantation (EP-A-10 70 490; EP-A-567349). However, these are ribs which, because of their shape, are unsuitable for displacement and compression of bone material and therefore require a shaping of the cavity such that their volume and their shape are taken into consideration from the outset in the cavity. Relatively wide shaft projections broadening from distal to proximal are also known for prosthesis shafts which are intended to be implanted using cement and in which, therefore, the associated tool is designed such that the artificial bone cavity is more voluminous than the shaft inclusive of its rib-like attachments.